Pressure transmitter calibration is what you need to do before you install the pressure transmitters. Also called pressure transducer calibration, or pressure sensor calibration.
In this article, we will share pressure transmitter calibration using hart communicator.
Pressure transmitters used in the process industries are very durable and reliable instruments.
Even so, they still require periodic maintenance and calibration to ensure optimal performance.
Before we start to calibrate the pressure transmitter, we should know:
What is span in pressure transmitter?
Fig. showing span and zero adjustment
Span value:
The difference between two minimum value and maximum value of readings is known as a span value.
As shown in fig. below span = 20mA – 4mA
Zero Value:
The value of readings at zero lines (Y-axis) is known as zero value as shown in the figure.
How Often Should You Calibrate a Pressure Transmitter?
Pressure transmitters require regular maintenance and calibration to ensure optimum performance.
There are no specific rules for the calibration of pressure transmitters. However, this depends on the regulations the company must comply with and the purpose of the calibration. Examples include safety specifications, application requirements, process conditions or as part of standard maintenance.
General industry practice is to calibrate pressure transmitters every 1 to 3 years based on the above conditions.
If it is found that there are obvious errors, or it is more important, the calibration cycle can be shortened.
Extended reading: Pressure Sensor Applications-Featured Industry Applications
how to calibrate a 4-20mA pressure transmitter
Once you have established the calibration interval and MPE, you are ready to perform the actual calibration procedure on your pressure transmitter.
The best-practice recommendation is:
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Mount the transmitter in a stable fixture free from vibration or movement.
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Exercise the sensor or membrane before performing the calibration.
This means applying pressure and raising the level to approximately 90 percent of the maximum range.
For a 150 psi cell that means pressurizing it to 130–135 psig. Hold this pressure for 30 seconds, and then vent.
Your overall results will be much better than if you calibrate “cold.” cent of the maximum range.
For a 150 psi cell that means pressurizing it to 130–135 psig.
Hold this pressure for 30 seconds, and then vent.
Your overall results will be much better than if you calibrate “cold.” -
Perform a position zero adjustment (zero the transmitter).
This is important because the orientation of the fixture used for calibration may be different than the way the transmitter is mounted in the process.
Failing to correct for this by skipping this step can result in nonconformance.
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Begin the Pressure Transmitter Calibration procedure.
Typically this means three points up (0 percent/50 percent/100 percent) and then three points down.
The 4–20 mA output should be 4 mA, 12 mA, and 20 mA at the three points (or the correct digital values for a smart transmitter).
Each test point should be held and allowed to stabilize before proceeding to the next.
Normally that should take no more than 30 seconds.
You can use more points if you require higher confidence in the performance of the instrument. -
Compare the results of your pressure transmitter to your reference device.
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Document the results for your records.
Pressure transmitter calibration formula
There is a formula that we can easily use to convert most (or all) units utilizing 4 to 20 mA signal to mA units.
There are others out there but this is the simplest I know.
Below is a simple formula for pressure to current conversion.
For example:
the range is : 0 to 10 Bar
Full range = 10 Bar
Displayed or measured value: 7 Bar
15.2 mA is the equivalent current value of a 7 Bar pressure.
（Read more about: Common Units Of Pressure）
For Value or range which is not starting with zero ( with a vacuum range), use below linear interpolation formula.
You can also encode this to excel for easier conversion.
If you want to know and calculate the error,
Just subtract the True value with your computed value.
Error = Measured Value – True Value.
If the Pressure Transmitter has an accuracy of 0.5% of the range,
then 0.005 x 7= +/-0.035 Bar,
you can use this as the tolerance to determine a pass or fail result.
Or you can ask the user for their respective tolerances.
Read more about: What Is 0-10V Signal Output?
How to calibrate pressure transmitter with hart communicator
Equipment required for Pressure Transmitter Calibration
Pressure transmitter, multimeter, HART communicator
The basic procedure for Pressure Transmitter Calibration
- Isolate the Pressure Transmitter from the Process.
- Slowly open the vent plug and the vent valve to release the pressure.
- Connect the multimeter with the transmitter and ensure that output is 4ma when 0 pressures are applied.
- Connect the handheld test pump (pressure source) to the transmitter.
- Ensure there is no leak.
- Apply pressure range at 0%, 25%, 50%, 75%, 100% and check there is any error.
- If there is any error calibration should be done.
Read more about HART Pressure Transmitter
If the transmitter is the analog transmitter
- Apply 0% pressure as per LRV with handheld test pump and check multimeter if it is not 4ma adjust the zero pot in the transmitter and correct transmitter output to 4ma
- Apply 100%pressure as per the URV and correct 20ma in multimeter by adjusting span pot in the transmitter
- Repeat these steps to rectify the error.
In case of SMART Transmitter
- We have to use HART communicator, connect the communicator with the transmitter select the HART Communicator Menu for lower range value trim and upper range value trim.
- Basic Set up – Calibration – Zero Trim/Sensor Trim —Lower/Upper range value trims.
- HART communicator will automatically calibrate the transmitter.
- Restore the process connection
- Take the transmitter on line. Ensure there is no leak
a small example of five-point calibration is given below
Low range value=0psi
upper range value=200psi
This calibration can work for Rosemount 3051 calibration.
Preparing for Field Calibration of Differential Pressure Transmitters
The usual practice is to disassemble the joint of the pressure guiding tube and the differential pressure transmitter, and then connect to the pressure source for calibration. It is troublesome and labor-intensive. The most worry is that there will be leakage or the pressure guiding pipe will be broken when disassembling and assembling the joint.
No matter what type of differential pressure transmitter, the positive and negative pressure chambers have exhaust, drain valves or cocks. This provides convenience for on-site calibration of the differential pressure transmitter, so that it can be calibrated without removing the pressure guiding tube. Differential pressure transmitter.
But make a fitting with the same thread as the vent, drain valve or cock.
When the differential pressure transmitter is calibrated, first close the positive and negative valves of the three-valve group. Open the balance valve, and then loosen the exhaust and drain valves to vent.
Then use a self-made connector to replace the vent, drain valve or cock connected to the positive pressure chamber.
The negative pressure chamber is kept unscrewed, allowing it to vent to the atmosphere.
The pressure source is connected with the self-made joint through the rubber tube. Close the balance valve. And check the air circuit sealing.
Then connect the ammeter (voltmeter) and the hand-operated communicator into the differential pressure transmitter circuit, and start the calibration after power-on and preheating.
Field Calibration of Conventional Differential Pressure Transmitters
First adjust the damping to zero state, first adjust the zero point. Then add full pressure to adjust the full scale, so that the output is 20mA. The adjustment should be fast in the field. Here is a quick adjustment method for zero point and span.
When the zero point is adjusted, it has almost no effect on the full scale, but when the full scale is adjusted, it has an effect on the zero point. When there is no migration, the effect is about 1/5 of the range adjustment amount, that is, the range is adjusted upward by 1mA. The zero point will move upward by about 0.2mA ,vice versa.
E.g:
The input full scale pressure is 100kPa, the reading is 19.900mA.
The range-adjusting potentiometer makes the output 19.900+(20.000-19.900)×1.25=20.025mA, and the range increases by 0.125mA. Then the zero point increases by 1/5×0.125=0.025, and the zero-point potentiometer makes the output 20.000mA.
After the zero point and full scale adjustment are normal, check the middle scales, and make fine adjustments if they are out of tolerance. Then carry out the adjustment work of migration, linearity and damping.
Smart Differential Pressure Transmitter Field Calibration
The intelligent differential pressure transmitter is between the input pressure source and the output 4-20mA signal. In addition to machinery and circuits, there is also a microprocessor chip that operates on the input data.
Therefore, the field calibration method of intelligent differential pressure transmitter is different from that of conventional differential pressure transmitter.
Read more about: Static Pressure Vs Dynamic Pressure Vs Total Pressure
The differential pressure liquid level transmitter has been calibrated according to customer requirements in terms of range, accuracy, linearity and other parameters. And mark the range, accuracy, etc. on the nameplate of the differential pressure liquid level transmitter. As long as the parameters such as the density of the measured medium meet the requirements of the nameplate, there is usually no need to adjust.
If the customer needs to adjust the span or zero position, please adjust according to the following methods. Assuming that the range of the differential pressure liquid level transmitter is 0~10 meters:
- Unscrew the back cover of the differential pressure liquid level transmitter, connect an external standard 24VDC power supply and an ammeter (requires an accuracy of 0.2% or higher) to adjust.
- When there is no liquid in the differential pressure liquid level transmitter. Adjust the zero point potentiometer so that the output current is 4mA.
- Pressurize the differential pressure liquid level transmitter to the full scale (10 meters). Adjust the full-scale resistor so that the output current is 20mA.
- Repeat the above steps two or three times until the signal is normal.
- Please input 25%, 50% and 75% respectively to check the deviation of the differential pressure liquid level transmitter.
- For non-water media, when the differential pressure liquid level transmitter is calibrated with water, it should be converted according to the pressure generated by the actual use of the medium density. For example, when the density of the medium is 1.3, the 1.3m water level should be used to calibrate the 1m range.
- After adjustment, tighten the back cover.
- The calibration cycle of the differential pressure liquid level transmitter is once a year.
- The HART intelligent differential pressure liquid level transmitter of Sino-Inst can be selected, which is convenient to adjust the range of the differential pressure liquid level transmitter.
Learn more about Pressure Transmitter Calibration
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Conclusion:
It is normal for the pressure transmitter to have a certain error. But if the error is too large, it needs to be calibrated. There are two types of Pressure Transmitter Calibrations: conventional method and intelligent calibration. no matter where
Kinds of preparations must be done before calibration, and then calibrate and debug through the handheld operator.
There are no mandatory fixed requirements for Pressure Transmitter Calibration. Generally, enterprises can formulate them by themselves. Normally, they can be calibrated once a year. Crucially, the calibration cycle can be shortened.
About how to calibrate the pressure transmitter, and what needs to be paid attention to during the process of Pressure Transmitter Calibration. If you still have questions, please feel free to contact our engineers.
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Wu Peng, born in 1980, is a highly respected and accomplished male engineer with extensive experience in the field of automation. With over 20 years of industry experience, Wu has made significant contributions to both academia and engineering projects.
Throughout his career, Wu Peng has participated in numerous national and international engineering projects. Some of his most notable projects include the development of an intelligent control system for oil refineries, the design of a cutting-edge distributed control system for petrochemical plants, and the optimization of control algorithms for natural gas pipelines.